Types of Solid Propellants
There are two basic types of solid-propellant charges-restricted burning and unrestricted burning. A restricted-burning charge has some of its exposed surfaces covered with a liner or inhibitor (view A in fig. 9-21). This covering confines the burning area and aids
Figure 9-21.-Solid propellant grains: A. Restricted burning; B. Restricted bored; C. Unrestricted burning; D. Grain patterns.
in controlling the burning rate of the propellant. The use of an inhibitor lengthens burning time and helps to control combustion-chamber pressure.
A restricted-burning charge is usually in the shape of a solid cylinder. It completely falls the combustion chamber and burns only on its end. The thrust developed is proportional to the cross-sectional area of the charge. Burning time is proportional to the charge length. The restricted-burning charge provides a low thrust and long burning time. Normally, it is used in the sustainer section of the propulsion system of the missile.
Unrestricted-burning charges are designed so they bum on all surfaces at once. The charge is usually hollow and bums on both the inside and outside surfaces. (See view C of fig. 9-21.) Since the inside area increases while the outside area decreases during combustion, a constant burning area is maintained. For an unrestricted-burning charge, thrust is also proportional to the burning area. The burning time of hollow grains depends on their web thickness. That is the distance between the inside and outside surfaces. An unrestricted-burning charge delivers a lot of thrust for a short period of time. It normally is used in the booster section of the propulsion system of the missile.
Certain SMS missiles use a separate
missile-booster combination. The solid-fuel booster, using an unrestricted-burning charge, provides the initial large thrust for a short period of time. In doing so, it gets the missile off the launcher rail, up to flight speed quickly, and extends the range of the weapon. The solid-fuel sustainer of the propulsion system of the missile uses a restricted-burning charge. It is ignited at booster separation and provides the low thrust, long burning time to "sustain" or keep the missile going down range.
Other types of SMS missiles use what is called a dual-thrust rocket motor (DTRM) (fig. 9-22). The solid-fuel propulsive charge is formed by bonding two types of propellants into a single unit. The center (booster) grain is an unrestricted charge and boosts the missile into flight. The outer (sustainer) grain is a restricted charge and sustains the missile until the end of flight.
Burning Rate of Solid Propellant Grains
The key point to understand about the restricted and unrestricted charges is that their burning rate is controlled. An uncontrolled burning rate would result in an explosion. That is fine for the warhead which we'll discuss next, but for a rocket motor. . . it could really ruin a paint job on the launcher.
The ideal solid-propellant would be ignited easily and continue to bum evenly. However, "ideal" is not possible. One way to control a burning rate is to use an inhibitor. An inhibitor is any substance that interferes with or retards combustion. The lining and washer shown in views A and C, respectively, of figure 9-21 are two examples of inhibitors.
Figure 9-22.-Cutaway view of a dual-thrust rocket motor.
another method of controlling the burning rate of a propellant is to use various grain shapes. Common examples of these shapes are shown in view D of figure 9-21. Resonance rods, mentioned earlier, may be used to offset the resonant burning or "chugging" of a propellant. These metal or plastic rods are sometimes included in the combustion chamber. They serve to breakup regular fluctuations in the burning rate and accompanying pressure variations. They do so by maintaining a constant burning area while the surface of the grain is being consumed.
The burning characteristics of a solid propellant depend on various factors. Examples include its chemical composition, initial temperature, size and shape of the grains, and so forth. In most missiles, the propellant is case-bonded to the combustion chamber walls. This bonding means the propellant composition is melted and then poured or cast directly into the chamber. This technique makes full use of the entire chamber area.
One limitation to solid-fuel propellants is their sensitivity to temperature changes. The burning rate of the propellant can be affected. A particular grain may produce more thrust on a hot day than it will on a cold day. Now, this doesn't mean you can't fire missiles on your next North Atlantic cruise (for you East Coast Sailors). However, it is a factor to be considered in the fire control problem.
Temperature also affects the physical state of a solid-fuel propellant. At extremely low temperatures, some grains become brittle and tend to crack. Cracks increase the burning area surface leading to an increased burning rate and combustion-chamber pressure. If the pressure exceeds the design strength of the chamber, the missile could explode. Cracks in the propellant grain resulting from the missile being dropped or jarred during handling will have the same effect-explosion.
High temperatures can make certain grains lose their shape and become soft and weak, possibly resulting in unsatisfactory performance. The optimum temperature ranges for most solid propellants in stowage is between 70°F and 100°F.
Since we are in the area of propulsion, it is appropriate to talk about acceleration as it affects a missile. Acceleration is a change in either speed or direction of motion. A missile experiences the forces of acceleration as it increases or decreases speed during flight, Changes in direction, dives, pullouts, and so forth, are also acceleration forces acting on a missile.
These forces are measured in terms of the standard unit of gravity. This unit is abbreviated by the letter g. A free-falling body is attracted to Earth by a force equal to its weight. As a result, it accelerates at a constant rate of about 32 feet per second. That is equal to one g. Missiles, making rapid turns or responding to major changes in propulsive thrust, experience accelerations many times that of gravity. The maximum g-force a missile can withstand determines the maximum turning rate of the weapon.